It isn’t every day that we learn about the discovery of an entirely new vector for an important vector-borne disease. A new report by the Australian Department of Agriculture and Fisheries has identified a new species of Leishmania that is transmitted by midges, not the usual vector, sandflies. Leishmania is a vector-borne protozoan parasite that causes an ulcerative disease known as Leishmaniasis or Kala-azar. Leishmaniasis is a disease primarily of the tropics and subtropics and is considered one of the most neglected infectious diseases in the world. The usual vectors are phlebotomine sandflies.
Australia (along with Antarctica) was thought to be the only continent free of Leishmania when locally-acquired infection was detected in kangaroos in Northern Territory in 2003. Researchers investigating this infection thought that the local sandflies (Sergentomyia spp.) seemed highly unlikely vectors because they show a strong preference for feeding on reptiles. Indeed, screening for Leishmania in 3046 Sergentomyia sandflies yielded none infected with Leishmania. This led the researchers to expand the vectors tested. What they found was an unnamed species of day-feeding midge (Lasiohelea sp.) that was infected with a prevalence of up to 15 percent. This is the first identified vector for Leishmania that is not a phlebotomine sandfly. Not much is known about this midge. The researchers were unable to find breeding sites, for example. The presence of prolegs on the midge larvae suggest that it is not aquatic but is terrestrial or semi-acquatic. The authors suggest looking for midge breeding sites in the moist soil near water troughs where kangaroos drink.
Finding a totally new vector for a disease carries with it implications for eradication and control. One possibility raised by this work is that the difficulty some control programs have experienced may reflect the fact that Leishmania is being transmitted by multiple vectors. This is an hypothesis well worth investigating in areas other than Australia.